Converter circuits are used nowadays in a wealth of power electronic applications. The requirements made of such a converter circuit here include, firstly, generating harmonics as little as possible on phases of an electrical AC voltage system conventionally connected to the converter circuit and, secondly, transmitting the largest possible powers with a smallest possible number of electronic components. A suitable converter circuit for switching a large number of switching voltage levels is specified in DE 692 05 413 T2, wherein a first switching group and n further first switching groups are provided for each phase, wherein n≧1 and the first switching group is formed by a first drivable bidirectional power semiconductor switch and a second drivable bidirectional power semiconductor switch and the n further first switching groups are each formed by a first drivable bidirectional power semiconductor switch and a second drivable bidirectional power semiconductor switch and by a capacitor connected to the first and second drivable bidirectional power semiconductor switches. Each of the first switching groups is connected in interlinked fashion to the respective adjacent first switching group, wherein the first and the second drivable bidirectional power semiconductor switch of the first further first switching group are connected to one another. The first and the second drivable bidirectional power semiconductor switches are each formed by a bipolar transistor having a drive electrode arranged in insulated fashion (IGBT—Insulated Gate Bipolar Transistor), and by a diode reverse-connected in parallel with the bipolar transistor.
A further generic converter circuit for switching a large number of switching voltage levels is specified in WO 2006/053448 A1.
What is problematic about a converter circuit for switching a large number of switching voltage levels according to DE 692 05 413 T2 is that the electrical energy stored in the converter circuit during operation is very high. Since the electrical energy is stored in the capacitors of the n first switching groups of the converter circuit, the capacitors have to be designed for this electrical energy, that is to say with regard to their dielectric strength and/or their capacitance. However, this necessitates capacitors having a large structural size, which are correspondingly expensive. Moreover, the converter circuit requires a large amount of space on account of the capacitors that are large with regard to the structural size, with the result that a space-saving construction such as is required for many applications, for example for traction applications, is not possible. Furthermore, the use of the capacitors that are large with regard to the structural size brings about a high mounting and maintenance outlay. Furthermore, the converter circuit for switching a large number of switching voltage levels according to DE 692 05 413 T2, on account of the exclusive use of bipolar transistors having a drive electrode arranged in insulated fashion as drivable power semiconductor switches, is susceptible to high voltages, in particular to overvoltages, and additionally has considerable active power losses.